Optical logic systems and the components combined to form them have represented subjects of increasing inquiry and interest on the part of the scientific community. This interest stems principally from the promise which such systems hold as an approach for significantly increasing the average processing speeds of digital data processing systems and in minimizing machine volume. In this regard, it is opined that the computational capacities of a computer might be enhanced considerably by resort to optical-digital computing systems having a capability permitting a simultaneous parallel processing of information introduced to them. Optical data processing utilizing coherent light has been extensively studied essentially since the evolution of laser. However, practical approaches for implementing such concepts have suffered severe constraints. For example the technique has represented only a special purpose processing capability, principally inasmuch as, in the past, it could not be performed in real-time. The speed of data through-put is important and such systems, historically relying upon photographic film for inputting data and for spatial filtering, have been off-line processes, thus, limiting flexibility to a great extent. Early concepts relating to coherent optical processing are presented, for example, in the following representative publications:
I. E. L. O'Neill, I. R. E. Trans. on Infor. Theory, IT-2, Pg. 56 (1956). PA1 II. P. Elias, et al., J. Opt. Soc. Am. 42, Pg. 127 (1952). PA1 III. L. Cutrona, et al., IRE Trans. on Infor. Theory, IT-6, Pg. 386 (1960). PA1 IV. D. P. Jablonowski and S. H. Lee, J. Opt. Soc. Am. 63, 1306 (1973). PA1 V. S. H. Lee, Opt. Eng. 13, 196 (1974). PA1 VI. N. C. Gallagher, Appl. Opt. 15, 882 (1976). PA1 VII. H. Wieder and R. V. Pole, Appl. Opt. 6, 1761 (1967). PA1 VIII. P. DeSantis, F. Gori, G. Guattari, and C. Palms, Opt. Ata. 23, 505 (1976). PA1 IX. P. W. Smith and E. H. Turner, Appl. Phys. Lett. 30, 280 (1977). PA1 X. J. Grinberg, A. Jacobson, W. Bleha, L. Miller, L. Fraas, D. Boswell and G. Myer, Optical Engineering Vol. 14 No. 3 (1975) PA1 XI. Photoactivated Birefringent Liquid-Crystal Light Valve for Color Symbology Display; J. Grinberg, W. Bleha, A. Jacobson, A. Lackner, G. Myer, L. Miller, D. Margerum, L. Fraas and D. Boswell, The Institute of Electrical & Electronics Engineers, Inc. (1975). PA1 XII. AC Liquid-Crystal Light Valve; T. Beard, W. Bleha & S. Wong, Appl. Phys. Lett., Vol. 22, No. 3 (1973). PA1 XIII. Spatial Light Modulators; D. Casasent, Proceedings of the IEEE (1977). PA1 XIV. Imaging Characteristics of the Itek PROM; S. Lipson & P. Nisenson, Applied Optics, Vol. 13, No. 9 (1974).
Current inquiries into optical information processing, in general, have looked to the use of optical feedback to evoke a variety of operations. As applied to purely electronic systems, feedback is well recognized as a procedure wherein there is combined a portion of the time varying output signal from a circuit (often after modification) with the input to that circuit to achieve a unique, temporal, frequency response characteristic. As applied to an optical information processing system the feedback procedure combines a part of the output image, usually as a two dimensional distribution of optical amplitude or intensity, typically modified, with the input image to produce a spatial as opposed to temporal frequency transfer characteristic. Optical feedback also can be used to provide nonlinear or space-variant optical processing. Over the relatively recent past, direct optical feedback in coherent optical spatial systems has been used for the purpose of image restoration, contrast control, analog computation and amplitude, as well as phase control in spatial filtering procedures. In the latter regard, reference is made to the following publications:
Of the above scientific endeavors, analog computation techniques are considered to be those not concerning discrete intergers but, rather, continuous operations such as those involved in taking the Fourier transforms of object images so as to reduce them to their harmonic content. The topic of optical feedback also has been considered somewhat generally in the linear systems approximation as set forth in the following publication:
Basic to all practical digital or discrete integer computational systems are bi-stable or multi-stable elements which operate in concert to treat an inflow of data in pulse form. Optical bi-stability on a single spatial element has been demonstrated by means of hybrid optical-electronic feedback to an electro-optic element inserted in a Fabry-Perot resonator. Such devices generally are limited in consequence of their capability for treating only a singular optical channel or path. To achieve a desired significant increase in information processing capability, a parallel channel optical system is required wherein digital related arithmetic operations are carried out on a simultaneous basis. To achieve this significant enhancement of processing capability, bi-stable or multi-stable system elements are required wherein no, relatively slow, electronic modulation is required within the feedback or related paths of their structures. A discussion concerning the use of electronic feedback within bi-stable single spatial elements is provided in the following publication:
In the past, a device somewhat representing the optical analog to a transistor or vacuum tube has been developed. Operative with coherent or noncoherent light, these devices have been generically termed: "Spatial Light Modulators" (SLM). One such spatial light modulator consists of a sandwich of thin films upon a supportive substrate which electrically control the optical birefringence of a thin liquid crystal layer. The device is solid-state in nature and compact requiring a low, a.c. power input and is fabricable, for the most part, utilizing thin film technology. In its general operation, an input or writing light to one face of the device at a given resolution cell or pixel thereof, depending upon the voltage imposed thereupon, alters the optical birefringence of its output. Inasmuch as a relatively great number of minute channels or pixels are defined about the surface of the SLM, it is capable of simultaneously actively operating along each of those channels. Other SLMs are available as active elements which operate upon incident light in alternate ways, for example, by varying the amplitude of incident light in response to a controlling light input. For further information concerning spatial light modulators, reference is made to the following publications: